This invention is directed to insulated, conductive through-vias formed in a self-supporting carrier substrate. The substrate must be thick enough to be self-supporting. Fabrication process practicalities place lower limits on via diameters. For realistic substrate thickness (e.g. 100–700 micrometers) and realistic via aspect ratios (e.g. 3:1 to 10:1), via diameters must range from about 10 micrometers to greater than 200 micrometers. Via fill methods common to integrated circuit interconnects become technically problematic or impractical when trying to fill such large blind holes (to be opened later). Ser. No. 10/729,254 and Ser. No. 10/700,327 overcome these practical issues by teaching a multitude of different techniques to fill vias with metal-ceramic pastes. However, paste-filled vias are associated with an undesirably high degree of porosity. Extensive processing is required to seal any via surface containing exposed porosity.
More specifically, this invention relates to filling and capping deep, high aspect ratio vias by methods that solve the problem of porosity. Since the vias can be hundreds of micrometers deep, it is important that the metal deposition rate be high. The only viable solid metal fill method currently available is laser assisted chemical vapor deposition (LCVD). Electroplating are metal-deposition processes that tend to be relatively free of the problems of voiding and porosity. Metals such as gold, silver, copper, and aluminum are commonly deposited by electroplating. However, no electroplating or electroless processes presently exist for W, Mo, or Ta. Plasma jet processes are available to deposit W, Mo, and Ta. Plasma jet processes are unsuitable for filling high aspect ratio vias because their high deposition rates, result in porous metal structures.
U.S. Pat. No. 4,938,996 teaches the use of laser assisted chemical vapor deposition of aluminum or refractory metals (e.g., tungsten) in vias. Discussion of this prior art and the present invention is below.
FIG. 1 is a description of the sample configuration required to practice prior art U.S. Pat. No. 4,938,996. FIG. 2 is a description of a sample configuration of the present invention. In chemical vapor deposition (CVD) a solid material is deposited from gaseous reactants by chemical reactions that occur on, or in the vicinity of, a surface heated to a critical temperature. In laser assisted CVD (LCVD) a laser is used as a localized heat source to heat the surface on which it is desired to deposit the solid material.
FIG. 1 shows a silicon substrate (100) is covered with a thin layer of SiO2 (110). The layer of SiO2 (110) covers the field of the substrate where deposition of metal is not desired. The SiO2 layer (110) functions to keep the field surface at a temperature below a critical temperature so as not to cause deposition of metal during laser heating of the Si substrate covered by layer (110). The thickness of SiO2 layer (110) is determined by the required metal feature height. Via (160) is made in SiO2 (110) and opens to a conductive layer (150), either doped Si or metal, previously deposited on the substrate (100).
The prior art laser radiates at frequencies that do not couple with SiO2. The laser must not couple with the SiO2 in order that the insulator not be heated in excess of a threshold temperature. Temperatures above a threshold, or critical, temperature causes metal deposition. Such heating of the SiO2 would lead to metal deposition at undesired locations. The prior art laser must further be a pulsed laser to avoid excessive conductive heating of the SiO2 coating on the surface.
The prior art laser couples with, and thereby heats, silicon (100) and the via metal (150). Because the laser must couple with the substrate in order to deposit metal thereon, the substrate is limited to materials such as Si.
In CVD processes, metal is deposited from a precursor gas. For tungsten, the most commercially viable precursor gas is WF6. Tungsten is deposited by reaction with a reducing gas, such as H2, as indicated in Scheme 1: 
The reaction product, gaseous HF, rapidly etches SiO2. Therefore, prior art processes do not permit tungsten to be deposited directly on SiO2, but are limited to deposition on Si or metal. An additional drawback of prior art processes is that SiO2 thicknesses and via dimensions are altered due to HF etching.
The prior art relates to small diameter, e.g. 0.5 μm, low aspect ratio, e.g. 1 to 2 μm, vias.